# std::ranges::views::keys, std::ranges::keys_view

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
template< class R > using keys_view = ranges::elements_view<R, 0>; | (1) | (since C++20)
namespace views { inline constexpr auto keys = ranges::elements<0>; } | (2) | (since C++20)

Takes a `view` of tuple-like values (e.g. std::tuple or std::pair), and produces a view with a value-type of the first element of the adapted view's value-type.

1) An alias for ranges::elements_view<R, 0>.
2) RangeAdaptorObject (and also RangeAdaptorClosureObject). The expression views::keys(e) is expression-equivalent to keys_view<views::all_t<decltype((e))>>{e} for any suitable subexpression e.

### Notes

keys_view can be useful for extracting keys from associative containers, e.g.

    std::map<std::string, int> map{{"one", 1}, {"two", 2}};

    for (auto const& key : std::views::keys(map))
        std::cout << key << ' ';
    // prints: one two

### Example

Displays values for each type of quark in particle physics.

Run this code

    #include <iomanip>
    #include <iostream>
    #include <locale>
    #include <ranges>
    #include <string>
    #include <tuple>
    #include <vector>

    int main()
    {
        const std::vector<std::tuple<std::string, double, bool>> quark_mass_charge
        {
            // name, MeV/c², has positive electric-charge:
            {"up", 2.3, true}, {"down", 4.8, false},
            {"charm", 1275, true}, {"strange", 95, false},
            {"top", 173'210, true}, {"bottom", 4'180, false},
        };

        std::cout.imbue(std::locale("en_US.utf8"));
        std::cout << "Quark name:  │ ";
        for (std::string const& name : std::views::keys(quark_mass_charge))
            std::cout << std::setw(9) << name << " │ ";

        std::cout << "\n" "Mass MeV/c²: │ ";
        for (const double mass : std::views::values(quark_mass_charge))
            std::cout << std::setw(9) << mass << " │ ";

        std::cout << "\n" "E-charge:    │ ";
        for (const bool pos : std::views::elements<2>(quark_mass_charge))
            std::cout << std::setw(9) << (pos ? "+2/3" : "-1/3") << " │ ";
        std::cout << '\n';
    }

Output:

    Quark name:  │        up │      down │     charm │   strange │       top │    bottom │
    Mass MeV/c²: │       2.3 │       4.8 │     1,275 │        95 │   173,210 │     4,180 │
    E-charge:    │      +2/3 │      -1/3 │      +2/3 │      -1/3 │      +2/3 │      -1/3 │

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3563 | C++20 | `keys_view` is unable to participate in CTAD due to its use of views::all_t | views::all_t removed

### See also

ranges::values_viewviews::values (C++20) | takes a `view` consisting of pair-like values and produces a `view` of the second elements of each pair (class template) (range adaptor object)
ranges::elements_viewviews::elements (C++20) | takes a `view` consisting of `tuple-like` values and a number N and produces a `view` of Nth element of each tuple (class template) (range adaptor object)
slice | BLAS-like slice of a valarray: starting index, length, stride (class)

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